The present invention pertains to an absorbent structure in an absorbent article such as a diaper, an incontinence protector, a sanitary napkin or the like, which structure exhibits two end portions and a crotch portion located therebetween, wherein the absorbent structure comprises superabsorbent material having a thermo-reversible liquid absorption capacity.
In current absorbent articles such as diapers, sanitary napkins or the like, only approximately 60% of the total liquid absorption capacity of the absorbent body is utilized. Therefore, it has long been a wish to be able to increase the degree of utilization of the absorption capacity in the absorbent body.
An absorbent body of an absorbent article exhibits two end portions, and a crotch portion located therebetween. The crotch portion is the part of the diaper which is intended to be arranged at the narrowest region between the thighs of the user during use, and is the area of the diaper which is expected to be wetted by excreted body fluid first. Since the area which the liquid reaches first is constituted by a relatively limited area of the total absorbent structure, this implies that the area is rapidly saturated with liquid. Therefore, it is important that the liquid rapidly can be distributed from this area to the remaining parts of the absorbent structure. However, a small portion of the liquid always remains within the area around the wetting site, without being able to be distributed further to the remaining parts of the absorbent structure. Therefore, it is of great significance that the area of the absorbent structure which initially is wetted has the ability to retain the liquid which is not distributed out into the absorbent structure, so that rewetting, i.e. passage of liquid back out from the article, is avoided.
In order to obtain high absorption capacity in an absorbent body, absorbent articles of the above-mentioned sort usually contain superabsorbent materials. Superabsorbents are polymers with the ability to absorb liquid in a quantity many times their own weight. Furthermore, they have the ability to retain absorbed liquid even when they are subjected to an external pressure. The efficiency of such superabsorbents is dependent on many factors, such as where and how the superabsorbent is mixed into the absorbent body, which physical form the superabsorbent has, and physical and chemical properties such as absorption rate, gel strength, and liquid retention.
A phenomenon which is referred to as gel-blocking may negatively affect the absorption capacity of a fibre structure containing superabsorbents. Gel-blocking means that the superabsorbent forms a gel during wetting which blocks the pores in the fibre structure and thereby renders the transport of liquid from the wetting area to the remainder of the absorbent body difficult. This results in a poor utilization of the total absorption capacity of the absorbent body and, furthermore, results in an increased risk of leakage.
In order to increase the degree of utilization of the total capacity of the absorbent structure, and in order to prevent gel-blocking, the absorbent structure usually exhibits a number of layers with mutually different properties with regard to, among other things, absorption rate, liquid distribution and liquid retention.
For instance, it is previously known through EP 0,202,125 to have a lower content of superabsorbent material in an upper liquid receiving layer of an absorbent body, i.e. a layer intended to be facing a user, than in a lower part of the absorbent body. Accordingly, the liquid passes through the receiving layer without being absorbed by this and is brought further downwards in the absorbent structure. Thus, an advantage with this design is that the liquid is stored in the lower part of the absorbent body, which implies that the upper layer, closer to the user, substantially remains free from liquid. Another advantage of this previously known design is that the liquid also has time to be distributed from the wetting site out to the end portions of the article, since it takes a certain time for the liquid to reach the superabsorbent. This results in a higher utilization of the total liquid absorbing capacity of the absorbent article. However, a problem with such a design is that there is a risk that the upper absorbent structure is perceived as wet when repeated wettings occur, since it only has the ability-to retain a limited liquid quantity.
Furthermore, it is previously known through EP 0,343,941 to build up the absorbent body of three different layers, namely, a receiving layer, a liquid-distributing layer and a liquid-storing layer. The first layer, the receiving layer, consists of a hydrophillic structure with a wetting area which has a lower density than the remaining areas of the receiving layer. By means of this design of the absorbent body, with a more open structure having a lower density in the wetting area that in the remaining absorbent structure, a rapid liquid absorption of large liquid quantities is obtained within the wetting area. Thereafter, the liquid is distributed via the liquid-distributing layer and is absorbed by the liquid-storing bottom layer. With such a design of the absorbent body, it is true that a relatively rapid liquid admittance is obtained. On the other hand, the problem remains that the upper structure in the wetting area of the article has difficulty in retaining liquid when subjected to repeated wettings, something which results in increased rewetting and, accordingly, an increased risk of a wet surface closest to the user.
Still another disadvantage with the previously known absorbent structure is that it is complicated to manufacture since it consists of several different materials which have different densities in different portions.
However, by means of the present invention an absorbent structure of the sort mentioned in the introduction has been achieved, exhibiting a higher degree of utilization of the total liquid absorption capacity of the structure. By means of the invention an absorbent structure which exhibits an increased liquid-retaining ability in the wetting area of the article has also been achieved.
Furthermore, the risk of gel-blocking, with entailing impairment of the liquid distribution pattern and increased risk of leakage, is reduced by means of the invention.
According to the invention, this has been achieved by means of an absorbent structure comprising a superabsorbent material having a thermo-reversible liquid absorption capacity, located in one or several delimited areas in the absorbent structure.
According to a preferred embodiment, the superabsorbent material having a thermo-reversible liquid absorption capacity is primarily located in the crotch portion of the structure.
According to one embodiment, the thermo-reversible superabsorbent is constituted by a polymer, exhibiting a thermo-reversible liquid absorption capacity. A polymer which exhibits a thermo-reversible liquid absorption capacity is characterized in that the polymer changes geometrical configuration at a certain temperature, whereby the hydrophillic and hydrophobic groups of the polymer, respectively, take up another position, resulting in a substantial change of the liquid absorption capacity of the polymer. Accordingly, thermo-reversible polymers exhibit a cloud point at a certain temperature, Cp, at which the liquid absorption capacity of the polymer is changed. For those polymers which have been found to be suitable for the purpose, the cloud point Cp occurs within the temperature interval 30-37xc2x0 C., preferably within the temperature interval 32-35xc2x0 C.
The liquid absorbtion capacity-of the intended polymers is lower at a temperature above 32-35xc2x0 C., than at a temperature below 32-35xc2x0 C. Since the body fluid initially has a temperature which is approximately 37xc2x0 C., this implies that the body fluid, when reaching the absorbent structure, passes the thermo-reversible superabsorbent without being absorbed by it. The body fluid is distributed further on from the wetting site in the crotch portion of the diaper to the end portions of the absorbent structure, where the liquid is absorbed by conventional superabsorbent material, i.e. without thermo-reversible liquid absorption capacity. This results in an increased degree of utilization of the total absorption capacity of the absorbent body.
Thereafter, the thermo-reversible superabsorbent starts to swell when the body fluid has cooled down after a certain period of time to the cloud point of the superabsorbent, i.e. depending on the type of superabsorbent which is used, to a temperature around 32-35xc2x0 C. Accordingly, this implies that the thermo-reversible superabsorbent can absorb the remaining liquid quantity within the crotch portion. In this manner, the risk that the article is perceived as being wet after a first wetting is eliminated.
A further advantage with a thermo-reversible superabsorbent which is located in the crotch area, or above all, in a portion of the crotch area in close connection to the wetting site of the absorbent structure, is that the risk of gel-blocking in this area is almost completely eliminated. The reason for this is that the greater part of the liquid already has been transported to other parts of the absorbent structure when the thermo-reversible superabsorbent starts to swell.
When a second wetting of the absorbent structure takes place, the temperature once again rises around the wetting site, which causes the liquid-containing thermo-reversible superabsorbent to be emptied of liquid. Thereafter, the liquid is distributed to the absorbent structure outside the crotch area, where the liquid primarily is absorbed by conventional superabsorbent material without thermo-reversible liquid absorption capacity. When the temperature of the liquid remaining in the crotch area after some time has cooled down to a temperature of around 32-35xc2x0 C., the remaining liquid is absorbed by the thermo-reversible absorbent which is now emptied of liquid and therefore has the ability to swell once again. This ability to swell once more during the second wetting is of course also valid for subsequent wettings.
Since it takes some time before the liquid has cooled down to a temperature around 32-35xc2x0 C., when using thermo-reversible polymers it is suitable that the remaining absorbent structure, i.e. the end portions, are built up from a certain portion of conventionally absorbing material which has the ability to momentarily absorb liquid and retain the liquid independently of temperature changes.
According to one embodiment, the thermo-reversible superabsorbent is constituted by a cross-linked co-polymer of N-isopropylacrylamide. This polymer is used, for example, in pharmaceuticals, in so-called drug-release systems in order to control the absorption of substances in the body. The compound is further described in EP 0,693,508 as being useful within a number of different fields, where absorption of a liquid is desirable. This polymer exhibits a low absorption of body fluid at a temperature above 32-35xc2x0 C., while the absorption capacity is considerably higher when the temperature of the body fluid is below 32-35xc2x0 C.
According to another embodiment, the thermo-reversible superabsorbent is constituted by a cross-linked co-polymer which is built up from N-isopropylacrylamide and a vinyl monomer containing carboxylic acid, such as acrylic acid. Since the carboxylic acid in the acrylic acid is hydrophillic and charged, the total absorption capacity of the superabsorbent is increased. Still another advantage of the carboxylic acid is that it assists in raising the temperature at which the thermo-reversible superabsorbent starts to absorb liquid, i.e. in raising the cloud point, Cp, of the polymer. However, this implies that it is of essential importance that the content of the vinyl monomer containing carboxylic acid is not too high in the thermo-reversible polymer. A further advantage of a low content of vinyl monomer containing carboxylic acid is that the salt sensitivity of the thermo-reversible superabsorbent is reduced.
According to a further embodiment, the thermo-reversible superabsorbent is constituted by a cross-linked co-polymer which is built up from N-isopropylacrylamide and a polymerizable monomer containing sulfonic acid, such as for example monomers of AMPS, i.e. 2-acryloamido-2methylpropane sulfonic acid. An advantage with monomers containing sulfonic acid is that sulfonic acid is a stronger acid than carboxylic acid, i.e. has a lower pKa-value than carboxylic acid.
According to a further embodiment, the thermo-reversible superabsorbent is constituted by a cross-linked cellulose derivative, such as for example ethylhydroxyethyl cellulose. An advantage with a superabsorbent which is constituted by a cellulose derivative is that the absorbent is manufactured from a renewable material. Another advantage with such a material is that it is biodegradable.
In order to further facilitate the distribution of the liquid from the wetting site to other portions, according to one embodiment the absorbent structure exhibits a first and a second layer, wherein the first layer which is intended to be closer to the user during use exhibits a lower density than the second layer. Consequently, a rapid initial liquid absorption ability is obtained in the first, more open layer, and an increased liquid wicking force is obtained to the finer capillaries in the second layer. It is also possible to increase the liquid distribution to the end portions of the absorbent body by providing a first layer with compression lines which extend along the longitudinal direction of the article. Since the finer capillaries in the compressed portions of the absorbent body transport the liquid better than the surrounding portions of the absorbent body, the initially wetted area is drained more rapidly of excreted body fluid.
The invention also includes absorbent articles such as sanitary napkins, diapers, incontinence protectors, or the like, comprising an absorbent structure enclosed in a cover, which structure comprises superabsorbent material having a thermo-reversible liquid absorption capacity primarily located in the crotch portion of the absorbent structure.
According to one embodiment of the absorbent article, the superabsorbent material having a thermo-reversible liquid absorption capacity is primarily located in the part of the crotch portion of the absorbent structure which is located closest to the liquid-permeable cover layer. An advantage with this embodiment is that the body fluid is distributed from the part of the absorbent structure which is located closer to the user during use to the part which is further away from the user. In this way, the risk of a wet surface in direct contact with the skin is reduced.
According to still another embodiment of the absorbent article, the absorbent structure comprises a liquid-receiving layer located closer to the liquid-permeable cover layer and a liquid storage layer located closer to the liquid-impervious cover layer. In this embodiment, the superabsorbent material having a thermo-reversible liquid absorption capacity is substantially located in the liquid-receiving layer of the absorbent structure.